The thermal power of nuclear power plant primary loop is not allowed to exceed the limit, and turbine power depends on the thermal power at a certain extent. In this paper, the operating characteristic of primary loop and thermal system calculation method of secondary loop are combined, and a coupled method is provided to analyze the characteristics of primary and secondary loops of APIO00 nuclear power plant. A good agreement is obtained between the calculations and design parameters with a discrepancy less than O. 4%. The effects of load regulation and season on the operating characteristics are analyzed. The results indicate that the thermal power of primary loop varies with the turbine power due to the operat- ing mode of reactor following turbine. Moreover, the turbine power could be maintained by lifting the thermal power of primary loop. But the thermal power is limited under an allowed value, so the turbine power would decreases with the circulation water temperature when the thermal power reaches the limit.